The field of the invention is superconducting electric motors, specifically those that require that the superconducting material of the rotor be cooled, requiring the use of a cryogenic coolant supply system and a vacuum chamber.
Superconducting motors provide increases in power and efficiency over motors of a conventional, non-superconducting design. However, the use of superconducting materials presents obstacles that increase the complexity of the motor. The most significant impediment to the use of superconducting materials is temperature.
The current state of the art in superconductor motor technology is the use of what are referred to as high temperature superconductors (HTS) in the rotor of an electric motor. Despite their nomenclature, high temperature superconductors require an operating temperature in the range of 30K to 70K. This requires the use of a coolant system to deliver a low temperature coolant, such as liquid neon or gaseous helium, to the superconducting material. It also requires that the superconducting material be enclosed in a vacuum chamber to provide thermal insulation.
The fact that the superconducting material is contained in the rotor, which must be allowed to rotate, poses a significant problem for the creation and maintenance of a vacuum chamber. One way to obtain a vacuum in the rotor is to manufacture it as a sealed vacuum chamber. This approach does not require that the rotor be connected to an external vacuum pump during operation. However, it does require that the welds and joints be of a very high quality. In addition, the composite materials commonly used in high temperature superconductors have inherently high outgassing rates that rapidly compromise the vacuum level. This requires that the motor be stopped and the rotor vacuum chamber be pumped out periodically to maintain a sufficient level of vacuum.
The second way to obtain a vacuum surrounding the superconducting material is to enclose the entire rotor (and sometimes the stator) in a stationary vacuum chamber. This allows that vacuum space to be constantly pumped by an external vacuum pump to maintain the requisite level of vacuum. The major disadvantage to this approach is that it requires rotating vacuum seals for the rotor shaft. The cost and complexity of rotating vacuum seals increases as the size of the shaft increases. Therefore, for very large motors, the use of rotating vacuum seals becomes prohibitively expensive.
The present invention overcomes the cost and complexity associated with creating and maintaining a vacuum insulation about the superconducting rotor coils in electric motors with large rotor shafts by continually pumping out the vacuum space through a rotating vacuum seal that is smaller in diameter than the rotor shaft. By using seals that are much smaller than the size of the shaft support bearings, and that do not have to support high radial loads, seal life is improved, seal cost is reduced, and leakage is reduced. The vacuum chamber is attached to the rotor to rotate therewith. Because the diameter of the coupling is not dependent on the diameter of the rotor shaft, the shaft can be made as large as desired without incurring the cost and complexity of large vacuum couplings.
Specifically, then, the present invention provides a rotor for use with a superconducting electric motor. The rotor includes a rotor support shaft having an outer surface having a first diameter for receiving a support bearing and having an inner axial bore and a vacuum seal with an interface dividing stationary and rotating portion of the vacuum seal, the interface having a second diameter smaller than the first diameter. A superconducting rotor winding communicates with the rotor support shaft to rotate therewith and a vacuum jacket is attached to the rotor support shaft to surround the superconducting rotor winding thereby providing thermal insulation. The inner bore of the rotor support shaft communicates with an interior of the vacuum jacket and a non-rotating vacuum line communicates with the inner bore so as to provide a path of evacuation of the interior of the vacuum jacket through the inner bore into the vacuum line. The vacuum seal fits between the vacuum line and the inner bore with one of the stationary and rotating portions of the vacuum seal fitting against the vacuum line and one of the stationary and rotating portions of the vacuum seal fitting against the inner bore.
Thus it is one object of the invention to provide a means for continuously evacuating a running motor. The use of a vacuum seal with a smaller diameter than the motor shaft makes a continuous coupling between the rotor and an external vacuum pump more robust and less expensive.
The vacuum seal may fit against the inner surface of the inner bore and an inner periphery of the vacuum seal fits against an outer periphery of the vacuum line.
Thus it is another object of the invention to provide a coupling that fits unobtrusively within one motor shaft.
The inner bore may include a concentric partitioning tube having a central lumen leading to the superconducting rotor windings and the vacuum line may include an inner concentric cryogen supply line positioned so that when the vacuum line communicates with the inner bore, the cryogen supply line engages the central lumen of the partitioning tube and the vacuum line communicates with the space between the partitioning tube and the inner bore.
Thus it is another object of the invention to provide a continuous cryogen supply to a rotating rotor.
The cryogen supply tube overlaps with the partitioning tube to minimize conduction between the vacuum seal and the cryogen of the cryogen supply line. Both the vacuum line and the inner concentric cryogen supply line extend beyond the second seal and are joined at their edges to provide an extended thermal path between the cryogenic temperatures of the cryogen supply line and the second seal.
Thus it is another object of the invention to permit the use of vacuum seals that cannot function at cryogenic temperatures.
The foregoing and other objects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings that form a part hereof, and in which there is shown by way of illustration, a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference must be made to the claims herein for interpreting the scope of the invention.